Cholera remains a significant public health problem in Vietnam. During the late 1980's, the National Institute of Hygiene and Epidemiology of Vietnam developed a killed oral cholera vaccine modeled on a Swedish vaccine tested by Branch investigators in Bangladesh during the mid-1980's. A field trial of this vaccine in Hue, which enrolled 100,000 subjects, found the vaccine to confer about 60% protection against El Tor cholera, with similar efficacy in young children and in older persons. This earlier vaccine has now been augmented to include coverage against the newly recognized 0139 serogroup of V. cholerae. Randomized, placebo-controlled pretests of the safety and immunogenicity of this vaccine were conducted in adults and in children in Hanoi. Each pretest found the vaccine to be safe and to induce credible serum vibriocidal responses. Based on these favorable data, a large-scale field trial of vaccine effectiveness,enrolling over 300,000 persons was initiated in the coastal city of Khanh Hoa in March 1997. In March 1999 the study population was stratified according to study agents received in 1997, and based on hamlet of residence, re-ramdomized to receive a two dose regimen of vaccine or placebo. Surveillance of the trial population is now in progress, enabling estimation of the clinical effectiveness of the vaccine over two years following vaccination.